The literature on anencephaly in twins ascertained in series has been reviewed and some hitherto unpublished data presented. 1. Anencephaly rates in opposite-sexed twin pairs seem not to differ appreciably from those in related singleton births. 2. There is good evidence that the incidence of anencephaly is higher in same-sexed twin pairs than in opposite-sexed pairs. One would infer that anencephaly is commoner in MZ than DZ pairs: this inference is supported by some data on anencephaly in twins of which the zygosity has been diagnosed. It is estimated that members of MZ twin pairs are about 1 2/3 times as likely to be anencephalic as are other individuals. 3. Concordance rates are not low, as is sometimes suggested, but are of the same order as recurrence rates. 4. It seems likely that concordance rates are higher in same-sexed than in opposite-sexed pairs. This would be expected because of the sex-preferential nature of the condition, and the hypothesized higher incidence rates in MZ pairs.